A great many light spectra useful for vision are found in nature and are emitted from man-made devices. Light Emitting Diode (LED) devices and light sources exist that generate light for illumination of objects primarily for direct vision by the human eye. Product categories include lamps for architectural lighting, retail and office product lighting, body mounted illumination lights, flashlights, lanterns, reading lights, streetlights, medical and dental uses, home lighting, automotive lighting, and many other lighting products.
For reference, there are also illumination systems for electronic imaging sensors that function at the edge of human vision or well outside of the human vision spectral and intensity capability range such as infrared and thermal imaging or ultraviolet light imaging outside of the direct-imaging sensitivity range of normal biological eyesight.
In illumination systems, LED light sources can provide a high degree of longevity, reliability, low cost per radiant-watt of light, and a wide selection of spectral characteristics.
Illumination approaches to lighting for vision with LEDs frequently seek to enhance the green-yellow 530-570 nm spectral range because this spectral range provides the highest “specification value of lumens” for a given input electrical power or radiant power. Lumens has become a common metric for measuring light intensity, but this metric provides actual radiant power convolved with the CIE's assessment of the human eye's color sensitivity curve. The use of “Lumens” as a consumers' primary metric for assessing light output capability of lighting products has resulted in much of the LED lighting industry evolving toward light sources that maximize “Lumens” and not maximizing light output for quality of vision.
Warm-white light LEDs are an improvement on cool-white LEDs for general illumination because they increase the valuable red component of the light spectrum. These lamps usually utilize added orange and/or red emitting phosphor(s) (typically 590-620 nm emission peak phosphors) to the white LED phosphor(s) or add orange-red 620-630 nm dominant emission peak LEDs. An example of where warm-white or even pink-tinted LEDs are especially important for illumination is when meat or bakery goods are being presented to consumers. Many retail display cases that were initially converted to cool white LEDs to save energy resulted in customer and retailer dissatisfaction due to lack of sufficient red light in the spectrum making meats, fish, and breads appear unappealing, and therefore have been or are being changed to warm-white or even pink lights with a moderately high orange and red spectral content.
There are numerous special-application exceptions to the use of white light for illumination using LEDs. These applications include red LEDs being used for dark-adapted vision or signaling, blue and/or cyan and red LEDs for identifying blood, cyan LEDs for making certain patterns stand out, green lights for use in greenhouses so as to not initiate certain photoactivation processes, ultraviolet or deep violet LEDs for making objects to fluoresce and stand out, and a few other specialty applications. These are all narrow and specific lighting applications.
As discussed in the definitions hereinafter, “Lumens” is a derived unit of luminous power, providing a measure of the total amount of “visible” light emitted by a light source. Luminous power reflects the varying perception of the “average” human eye to different wavelengths based on the Luminosity Function standard established by the Commission Internationale de l'Éclairage (CIE) in 1931 for the human eye's sensitivity. This function provides an average spectral sensitivity of human visual perception of brightness based on a non-representative sampling of the human population. The Luminosity Function standard was determined using subjective judgments by selected participants who indicated which of a pair of different-colored lights is brighter, to describe relative sensitivity to light of different wavelengths. The CIE luminosity function is a standardizing function used to convert “radiant energy” into luminous (i.e., “visible”) energy.
Light that is close to or outside the CIE defined visible light spectrum limits of the 400 nm to 700 nm range provides virtually no measured Lumens, regardless of light's actual radiant power or the fact that most people can easily detect light wavelengths far beyond the CIE spectral range illuminating white surfaces even at well under 1 mW/cm2 radiance. These “outside of the CIE range” wavelengths are present in many types of non-LED light sources and some LED light sources, contributing useful radiant power in view of the fact that most people easily perceive these wavelengths (unless color blind to certain spectra). Incandescent light sources predominantly provide these near-infrared wavelengths of light as well as longer wavelengths far into the infrared. Visible light is defined as the approximate range within which most people can see reasonably well under most circumstances. Various sources consider visible light as broadly as from 375 nm to 800 nm. Under ideal conditions, many people can even visually perceive light from under-330 nm to over-950 nm. The vast range of objects in the world that are being illuminated reflect light or are activated to fluoresce over a wide range of range wavelengths unrelated to the CIE curve.
Human eyes are generally several times less sensitive to 390 nm-400 nm violet light or to 700 nm-730 nm deep-red light than they are to 530 nm-570 nm range yellow-green light. Illumination light intensity, the light source spectrum, light angles, polarization, reflected or emitted light intensity from objects of interest, background and surroundings, and individual perception of light spectra all effect people's ability to visually detect objects using a selected light source. There have been publications on the subject of human vision range showing actual human vision ranging from 330 nm up to over 1000 nm, but some of the more recent publications where extensive subject sampling was performed put “useful” human vision into at least the 375-750 nm range, depending on how the experiments were performed and the test subject criteria (illustrative references include (375 nm-750 nm): Curtis, Barnes. Invitation to Biology: Fifth Edition. New York: Worth Publishers, 1994: 163, and (375-780 nm): Lapedes, Daniel. Dictionary of Scientific & Technical Terms: Second Edition. New York: McGraw Hill, 1978: 954.)
Color Rendering Index (CRI) has recently begun to be emphasized for general LED lighting to correct for some of the problems originating from the extensive use of lumens as the only light output metric. Several versions of CRI as an added metric have been used for some time in photography, although there are also other standards. CRI provides a quantitative measure of the ability of a light source to “reproduce the colors of various objects” faithfully in comparison with their concept of an “ideal” or “natural” light source. Unfortunately, a variation of the flawed CIE assessment of human eye spectral range was also applied to CRI metrics in determining the spectrum of an “ideal” light source. Therefore, objects absorbing or returning light to the eye outside the CRI range are considered almost invisible, and can almost become invisible if illuminated using a light source providing a center of the CIE curve shaped light spectrum. While adding CRI as a metric is better than using lumens alone, and although CRI can be useful for display applications where subjects are effectively looking at the light source, CRI can also be very misleading if used in the selection of LED lighting for many non-reflective surface illumination applications.
If the eye is less sensitive to light in a portion of a spectral range, increasing the relative intensity of light in these lower optical sensitivity spectral ranges increases the probability of detection of objects that absorb or reflect primarily in such portion of the light spectrum. Many materials may also fluoresce in the visible range after absorbing light, affecting their visibility. The eye can perceive the light returned directly from the object, and light returned from other nearby or background objects. Coatings on objects such as water, oils, or wax can also influence the probability of visual detection of objects.
LEDs are available providing light spectra more closely approximating various embodiments of the “simulated-sunlight” spectrum, or mimicking incandescent light. Black body light spectrum or fluorescent lighting can provide a better range of colors and can make the color of objects appear more “normal”. Reasonable sunlight (“visible” part of the spectral range) and other recreations of natural spectra have been shown. Light spectra similar to sunlight can recreate and “resolve” colors as they would appear in sunlight, especially if the lamps also contain considerable ultraviolet light. The presence of UV in sunlight or a simulated spectrum can activate fluorescent effects in many materials that also greatly affect perceived color.
Most LED light source products in the market create white light consisting of one or more blue 440470 nm spectral range LEDs with one of more spectrum converters (usually phosphors) mixed into a translucent medium and placed over the LED, usually as a coating on the LED. This spectrum converter usually consists of YAG phosphors with predominantly 530-580 nm light emission peaks in a polymer, glass, or silicone medium. The phosphor spectrum converter may or may not have additional phosphors added. Red phosphors such as AlGaAs, GaAsP, GaN, GaP, and/or AlGaInP are frequently added to increase the red spectral content and create warm-white LEDs.
Single narrow spectrum wavelength LEDs such as red, blue, green, or cyan are used in many lamps as special vision modes or for entertainment, stage lighting, displays, backlighted control panels, signaling and flashing. Colored lights are seldom used for general illumination.
Mixing of different dominant emission peak types of LEDs is well known for tailoring specific light spectra. Red (usually this is an orange 620-630 nm dominant emission peak LED), green, and blue LEDs are commonly used at different relative intensities to create a wide range of colors by electronically control of fixed color light sources.
A great many LED configurations are known with the phosphors or dyes coated onto the LED or remotely placed in the exiting light path. Concentrations of spectrum converters in a medium, on a surface, or on a reflector can vary, depending on the length of the light path, the percentage of incoming light to be absorbed, and the output light spectrum that is desired.
Optical absorbing filters or reflecting filters (interference filters such as dichroic filters or absorbing-only dyes or pigments) are sometimes used to remove or reflect portions of the output light spectrum at the expense of losing the energy associated with the portion of the spectrum that is removed. Commonly seen examples of filtered light are in stage lighting, automobile tail and running lights, indicators and control panels.
Dyes, phosphors, and/or pigments are used as fluorescent, phosphorescent, and other spectrum converters in various media, to shift light spectra and reshape the light energy distribution in light spectra. These media containing spectrum-shifting materials have been shaped into lenses, coated onto reflectors, and formed into a wide variety of shapes in the light path. Photonic arrays such as photonic crystals and nanoparticles such as quantum dots or rods in or on various media or reflectors have also been used to modify light spectra with less energy loss than absorbing or reflecting only filters, or to shift a spectrum to longer wavelengths (or to shorter wavelengths using 2 photon materials).
Most white light LED lamps provide a spectrum of cool white or warm white light. These lamps contain blue light, and blue light that has been converted using phosphors to green light and to orange-red light wavelengths, with a small percentage of the light spectrum in the red. There is also usually a gap in the cyan (475-510 nm) spectral region of most white light LED lamps between the blue emission of the LED that is passed by the phosphor, and the phosphor emission peaks in the yellow-green, yellow, or orange spectral regions. A low radiance of 480 nm-510 nm cyan light is present in conventional white light LED lamps, relative to 550 nm-570 nm yellow or ˜450 nm blue. Since cyan is the spectral region where the eye's rods are most sensitive, this creates an issue that did not exist with natural light or most other pre-LED light sources. Red phosphors have become available using GaN that can provide deeper red such as 650 nm or 670 nm. Phosphors, dyes, and QDs are known that can provide long wavelength light emission. See, for example, the Intermatix and Bejing Yuji examples discussed elsewhere in this disclosure.
Full-visible-light-spectrum LED-based lamps have been demonstrated that simulate the profile of the sun's spectrum with a high relative green-yellow (520 nm-590 nm) light intensity (relative to other parts of the spectrum), utilizing multiple LEDs and phosphors and/or other spectrum converter materials. Examples of this approach are described in U.S. Pat. No. 5,998,925.
LED light sources using UV LEDs and other UV light sources have been used with phosphors and other color converters to generate violet light and light in most other portions of the visible light spectrum.
Many full-visible-spectrum lamps use multiple LEDs (including orange-red, red, green, and sometimes violet LEDs) and some phosphor coated LEDs. This approach has resulted in better lamps, but such lamps still have multiple weak intensity voids in the visible light spectrum, and/or require six or more types of LEDs to be mixed. Mixing large numbers of different LEDs can be costly, since different LEDs frequently also require different electrical bias conditions and the resultant level of assembly complexity can become very high.
LED lamps also are widely available that provide special colors or mixtures of different spectral ranges of light. Blood tracking lights are just one example of a special spectrum, and are designed to highlight fresh blood using a combination of blue and orange-red wavelengths of light. Examples are described in Fiskars U.S. Pat. Nos. 7,290,896; 7,517,307; and 8,113,681. Blood tracking lights use a mixture of different wavelength LEDs in red and blue, including cyan. These special-purpose lights typically contain very little light in several portions of the visible spectrum, and thus are not appropriate for maximum detection of the widest range of objects.
LED lamps that provide large amounts of blue light and red light relative to green-yellow light are used for horticultural lighting. These LED lights sometimes also include a few conventional white light LEDs, but their spectra usually contain very little green-yellow light since this spectral range is not significantly utilized by most plants. Therefore, most green plants appear almost black under these light sources. These lamps typically use six-or-more types of LEDs in combination and seldom contain color converters. All horticultural LED lamps have one or more very weak spots in visible spectrum, where the light intensity is less than 9% of the highest photon power in the lamps' spectrum. These spectral gaps are frequently intentional for energy efficiency or due to small spectral gaps that can occur between various narrow emission spectra LEDs.
The human eye's color-sensing cones are collectively most sensitive to the yellow-green part of the spectrum. The commonly used light output measurement system of lumens weighs this yellow-green spectral range light heavily. A focus on maximizing lumens, as a guide for illumination lamp development, is at least partly flawed. The search for lumens instead of radiant power leads LED makers to introduce or accept spectral weaknesses in the output visible spectrum of the lamps. Since a great many objects in the overall world are not mostly yellow-green reflecting, providing the maximum radiant power in the 530-570 nm spectral range is flawed for many vision-oriented illumination applications in which the illumination light intensity may be low and detection of the maximum number of object types is the only objective. Lumens as a metric is best suited for predicting relative brightness to the eye when looking directly at the light source, or when illuminating objects that are highly reflecting in the 500-600 nm spectral range.
Flat-spectrum LEDs and sunlight-spectrum simulating LED lamps partly address the issue of missing portions of the visible spectral range, since the entire visible spectrum may be provided. See, for example, U.S. Pat. No. 7,646,032, U.S. Patent Application Publication 20100289044, and U.S. Patent Application Publication US20130134885. However, these lamps provide excessive amounts of light energy in the yellow-green spectral range where the eye is most sensitive. Sunlight, flat spectrum, incandescent, and other LED light sources that simulate other common light sources are best when multiple people need to agree on the color of objects. However, these conventional light spectra are not the most energy-efficient solutions for basic visual detection of the widest range of objects and assessing their shapes in low light intensity situations such as are frequently encountered, especially when using small battery-operated mobile outdoor lighting devices (flashlights, lanterns, body mounted lights, bicycle lights, etc.).
Commercial white LEDs are weak in the 480 nm-500 nm cyan portion of the spectrum where the eyes' rods are most sensitive and used for dark-adapted vision (Scotopic vision), relative to the 530 nm-570 nm portion of the spectrum. The eyes' rods play a major role in visual acuity, especially in low light situations. The concentration of rods is highest in the macula of the eye. High night vision capability animals typically have higher concentrations of rods in their macula than humans. Cyan LEDs are primarily used for special purpose lighting such as all-cyan lamp forensics, blood lights with orange-red and blue LEDs, and in combination with red LED light for high visibility yellow traffic lights. Cyan LEDs are not used in white lighting, even though they can improve visual acuity. This oversight is attributable to a blind focus by lamp designers on only providing the most “lumens”, and not on providing the best quality of full spectrum light.
No light sources have been demonstrated that provide a full visible spectrum of light with over 1.1 times enhanced radiant power in both the deep-red and violet spectral intensity components relative to the 530-570 nm green-yellow part of the spectrum, and enhanced light intensity in the cyan 480-510 nm range that is greater than the highest radiant power anywhere in the 530-570 nm spectral range.
No light sources have been demonstrated that specifically target (1) providing light over the full visible spectrum from 405-730 nm, and (2) providing over 1.1-times enhanced higher relative light intensity specifically in the 405430 nm spectral range, the 630-700 nm spectral range, and the 470-510 nm spectral range, relative to the middle yellow-green 530-570 nm portion of the light spectrum.
No light sources have been demonstrated that specifically target (1) providing light over the visible spectrum from 440 to 730 nm or longer wavelengths, and (2) providing over 1.1-times relative light intensity specifically within the 630-730 nm spectral range, and the 470-510 nm spectral range, relative to the yellow-green 530-570 nm portion of the light spectrum.
Light sources with UV, violet, blue, and/or cyan LEDs have been disclosed that have phosphors and/or other color-converting materials in a wide variety of configurations. See, for example, International Publication WO2011120172A1 and Chinese Patent Publication CN 202871750 U.
Up to 85% and higher internal QY red phosphors are available, while lower cost red phosphors have under 60% internal QY. Light scattering by phosphor particles and absorption losses can cut the output light efficiency by another 20% to 50%, depending on the amount of phosphor used, the medium, and the overall design used due to internal scattering by phosphor particles, trapped light, misdirected light, and other losses. QDs and some other nanoparticles can provide higher internal conversion efficiency and are less scattering-prone than standard phosphors, but also entail higher cost. Mixtures of different phosphors and/or QDs have been used to create multiple types of specific light sources.
Fluorescent dyes can have low internal scattering losses in many transparent media since the dyes are molecular in size (as long as dye aggregation is minimal). A few red-emitting dyes exhibit over 90% QY, and therefore dye-based lamp systems can be up to two times as efficient as typical particle phosphor systems, if over 80% of the light is to be converted to the orange and red spectral range, if the Stokes shift is adequate, if light trapping is controlled, and if the photostability of the dyes is adequate for the application. Fluorescent dyes are typically not used in commercial LEDs, as it has not been previously demonstrated how to accomplish these combined objectives using available dyes in reasonable media. Photostability issues are usually present, and the Stokes shift is usually small, so most red emitters tend to absorb poorly in the violet-blue where most of the more efficient LEDs reside, and because of the need for some blue light in most white light spectra. See, for example, International Publication WO/2012/042415, describing the use of dyes with LEDs.
Dyes may aggregate into groups of molecules, or bond with the medium used to contain it. Such effects typically reduce QY, and cause the dyes to remain dispersed rather than dissolved in a medium, so that the medium contains some combination of both dye aggregates and non-aggregated dye molecules. Proper dye-medium compatibility, dye-medium mixing, and processing of the dye-medium mix is important for the achievement of high QY.
Non-fluorescent dyes are sometimes used as filters to absorb portions of the light spectrum to provide more pure visual color, but absorption filtering wastes a significant amount of light energy and is therefore usually undesirable for practical applications unless it is to block UV or long wavelength IR, or otherwise represents the only way to achieve a desired light spectrum at reasonable cost.
Biological applications for fluorescent dyes, quantum dots, and phosphor-like nanoparticles have included tagging and tracking of biological materials, and use as photosensitizers or in photodiagnostic systems. Photoacoustic applications are also known.
The art therefore continues to face unresolved needs in the generation of visible light spectra, and particularly in the creation and use of such spectra to achieve improved visual acuity and enhancement of visualization processes.